Scale formation typically involves the precipitation and deposition of dense materials on surfaces made of metal and other materials. Scale formation may occur when inorganic mineral salts (including, for example, calcium carbonates, calcium sulfates, calcium oxalates and barium sulfates) precipitate from liquids and deposit on the surfaces of a system (including, boilers, evaporators, reactors, cooling water systems, heat exchangers, pipes, filter cloths, reverse osmosis membrane surfaces, oil wells, and desalination evaporators, among others).
Scale formation may cause a number of operation problems, including, but not limited to, plugging of equipment, pressure loss, increased utility costs, reduced heat exchange capacity, corrosion, lost production due to downtime, and downgraded products resulting from insufficient feeds. Scaling equipment may occur in a variety of industries. For example, in the petroleum industry, scale deposition costs millions of dollars every year and is one of the leading causes in production decline worldwide. Scale deposition is recognized as one of the top production problems in regions that are prone to scale, such as the North Sea, the United States and Canada.
Scale can be deposited in various equipment along various water paths, including, but not limited to, piping, injectors, reservoirs and surface equipment. Scale formation at oil-producing wells also may eventually result in lower oil yields and in well failure. Scale found in oil fields may form by direct precipitation from naturally-occurring water in reservoir rocks, or as a result of water becoming over-saturated with scale-forming species when two incompatible water streams combine. Furthermore, scale may also form when an oil or gas well produces water, or alternatively, when water injection is used to enhance recovery.
Natural water in oil fields may contain dissolved substances acquired through contact with mineral phases in the natural environment. Deep subsurface water may be enriched in soluble ions through alteration and dissolution of minerals. The water in sandstone reservoirs or geological formation water that have contact with sea water may contain abundant scale-forming ions, including but not limited to Ca2+, Mg2+, Ba2+, Sr2+, Na+, K+, CO32−, SO42−, and Cl−. Sea water is also generally rich in scale-forming ions, including ions that are by-product of marine life and water evaporation. In off-shore oil production, the formation of sulfate scales may occur when sea water, which may be rich with SO42− and formation water containing high concentrations of barium and calcium are mixed.
Furthermore, oil field scale may form when the state of any natural fluid is altered, such that the solubility limit for one or more components is exceeded. Temperature and/or pressure changes, pH shift, out-gassing, and/or the contact with incompatible water may cause the water to become oversaturated with scale-prone species and lead to the formation of scale.
Barium and strontium sulfate scales may, for example, be particularly troublesome when sulfate-rich seawater is used as an injection fluid in oil wells whose formation water is rich in barium ions. Barium and strontium sulfate generally form very hard, very insoluble scales that can be difficult to prevent by conventional chemical-based scale inhibition techniques. In some instances, this can be particularly troublesome, as barium and strontium sulfates can co-precipitate with radium sulfate, making the scale mildly radioactive. Dissolution of sulfate scales in piping is generally difficult to remove, possibly requiring one or more of high pH, long contact times, heat, high pressure, and high velocity circulation.
Barium sulfate, as well as other inorganic supersaturated salts, may precipitate onto the various formations to form a scale, thereby clogging the formations and restricting the recovery of oil from a reservoir. The insoluble salts may also precipitate onto production tubing surfaces and associated extraction equipment that may, for instance, limit productivity, limit production efficiency, and compromise safety. Certain oil-containing formation waters are known to contain high barium concentrations of 400 ppm and higher. Since barium sulfate forms a particularly insoluble salt, the solubility of which declines rapidly with temperature, it can be difficult to inhibit scale formation and to prevent plugging of the oil formation and topside processes and safety equipment.
Scale inhibitors can be used in production wells to prevent scaling in the formation and/or in the production lines down in the hole and at the surface. Scale build-up decreases permeability of the formation, reduces well productivity and shortens the lifetime of production equipment. Various scale inhibitors are known and include chelating agents, phosphates, phosphonates (organophosphates), polycarbonates, and components of polymers, have been developed to inhibit or reduce the formation of inorganic scales, as described for example in U.S. Pat. No. 7,943,058 to Hills et al., U.S. Pat. No. 7,491,682 to Gupta et al., and U.S. Pat. Pub. No. 2011/0089115 to Lu, the subject matter of each of which is herein incorporated by reference in its entirety. These scale inhibitors typically work by one of the following mechanisms: precipitation threshold inhibition, dispersion, and crystal distortion/modification.
Various methods are also known for introducing these scale inhibitors into production wells. For instance, a liquid inhibitor may be forced into the formation by application of hydraulic pressure from the surface which forces the inhibitor into the targeted zone. Alternatively, the delivery method may consist of placing a solid inhibitor into the producing formation in conjunction with a hydraulic fracturing operation.
Notwithstanding the use of scale inhibitors, once scale has deposited or formed on surfaces, it is necessary that such scale deposits be removed so that the equipment can continue to operate properly. However, strict environmental regulations for the North Sea, as well as many other areas, have required oil field service companies to formulate their products so that their effect on the marine environment is minimized and the components of the products comprises only approved components.
Using acids to dissolve calcium and other scale deposits in subsea environments is known. Calcium deposits build up on subsea components and eventually interfere with the operation of the components, including connectors and mechanical external valve components, from functioning. High pressure water jets and mild water jets are commonly used to clean equipment. The strength of the acid is also typically kept low to avoid corrosion and degradation of metallic and elastomeric materials on the subsea equipment and a constant flow of acid is required to maintain an acidic environment around the area being cleaned.
For use in the North Sea, chemical products are categorized into one of four categories based on the ecotoxicological properties of its components. The four categories, each designated by color, are as follows:                (1) Black—forbidden to use or discharge;        (2) Red—high priority for phasing out via substitution;        (3) Yellow—environmentally acceptable; and        (4) Green—only for chemicals listed on the OSPAR Convention for the Protection of the Marine Environment of the North East Atlantic PLONOR (Pose Little Or No Risk) database.        
Thus, for oil and gas drilling operations in the North Sea, companies are required to phase out the use of Black and Red components in their products and to use only chemicals which are entirely comprised of “Green” components in any new products.
Thus, there remains a need in the art for an improved composition for removing scale deposits from surfaces, especially in subsea environments that contains at least substantially only chemicals listed on the OSPAR Convention for the Protection of the Marine Environment of the North East Atlantic PLONOR (Pose Little Or No Risk) database.